What Apparent Wind Sailing Might Teach Us About Fins

So to start I should probably explain what “Apparent Wind Sailing” is. Everyone knows that in order to sail a boat you have to get the sail set at the right angle to the wind. However, the fun part of the problem is that, as the boat picks up speed, you have to change the angle of the sail to take account of the head wind created by the boat moving forwards.

The “True Wind” is the wind created by the weather, the “Induced Wind” is created by the boat moving forwards through the air, and the “Apparent Wind” is the Vector of the two.

If you look at this example, a 10mph wind is hitting the boat at 90°. The sail is set to the best angle for that and the boat then starts to move forward at 4mph, effectively creating a headwind. This means that there are now 2 winds hitting the boat. The “True Wind” of 10mph @ 90° and the “Induced Wind” of 4mph @ 0°. We must now set the sail on the boat to the vector of these two winds, which is called the “Apparent Wind” as it’s what we’d actually feel on our faces. The faster the boat goes, the more the Apparent Wind shifts away from the True Wind, towards the Induced Wind, and on modern high performance boats, this can cause problems. It’s why the America’s Cup boats often didn’t use their hydrofoils when sailing into the wind, as they’d go too fast and get stopped by their own headwind!

Team Oracle, slower upwind leg (L) and faster downwind leg (R)

So what can this teach us about surfboards?

Well as you may have noticed, the side fins on your surfboard have their leading edges angled slightly in, towards the nose of the surfboard (called “toe-in”) The angle varies from between 0° to 7° with around 2°- 5° being most common. The aim of this toe-in, is to set the fins at the best angle of attack to the water flow on the bottom of the surfboard. However, that flow is very similar to the wind on a sailing boat, in that it’s really a vector of our equivalent of the “True Flow”created by the wave, which we spoke about in my previous post about “How A Surfboards Works”, and the “Induced Flow” created by the surfboard moving forwards across the wave. If you now want to know the toe-in on your boards, try this cool online calculator.

The “true Flow” created by the wave moving through the water, and the “Induced Flow” created by the surfboard moving across the wave.

Now the interesting part to this is that we know the effect of adjusting the angle of toe on our fins, but we don’t really understand why these effects take place. If the fins are straight, with no toe-in, the board will tend to go fast and straight across the wave. The more the fins are towed in, the better the board turns, but the slower it goes in a straight line, and if the fins are toed in too much, the board will feel slow and unstable. There are several theories as to why this might be, but the problem is, there has been very little research to find out at what angle and speed the water is actually hitting the fins, and without that knowledge it’s hard to be sure.

One theory assumes that the water flow is spreading out from the centre of the board, and angling the fins inwards gives them the correct angle of attack to achieve laminar flow and create lift. A second theory assumes that the water flow is mostly inline with the stringer, and suggests that angling the fins inwards, would re-direct the water outwards, helping to break the rail out of the wave and initiate a turn..

THEORY A: The fins work as a hydrofoil, creating lift that would suck the board downwards in this image. THEORY B: The fins direct water outwards, pushing the tail up in this image

One of the only studies to address this subject was done by Beggs & French at the University of New South Wales, Australia in 2009, and showed that the fins are hit by water from many different angles, so that both theories may actually be correct. What does appear to be true, is that understanding the apparent water flow may have some important implications for the future of fin and surfboard design, and that’s why, when it looked like a work trip might take me to the Abu Dhabi wave pool a few years ago, I rigged up a board with some old fashioned “Tell Tales” in the hope of getting some underwater photos in the clear, predictable waves. Unfortunately the pool broke, and I’ve never had a chance to see if my plan would work. Maybe one day…

String “Tell Tales” attached to my small wave board in the hope of understanding the water flow a little better.